They purchased lifetime access to the content. Should I not give it to them just because they didn’t like an email once?

Imagine being grumpy at a Starbucks barista once and now you can’t get Starbucks again ever anywhere.

Or a better example: You opted out of email now you can’t reset your password. Sorry can’t email you the link.

Why would unsubscribing from your newsletter stop them from accessing the content? Is the content only delivered via the newsletter from which they unsubscribed?

> A while later /../ migrate to a new platform. I need to tell every buyer that their account is moving. But some have unsubscribed from all emails.

They unsub'd all meaning "no more emails ever" as you said. How do I tell them the stuff moved if they don't want to be contacted ever again ever?

You don't have to. When they can't find you, assuming they miss your content, they'll use this miracle tool called a "search engine" to find you.